<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<script>
    var a = null;
    console.log(a);
    console.log(typeof a);
    //undefined 和 null 进行相等比较时，结果为 true
    if (undefined==null){
        alert('aa');
    }
</script>
</body>
</html>